1. Field of the Invention
The present invention relates to a keyless entry system that locks or unlocks a door of a vehicle by performing wireless communication between an in-vehicle device and a portable device, and more particularly, to a passive keyless entry system having a function of automatically communicating between an in-vehicle device and a portable device.
2. Description of the Related Art
In a moving vehicle such as a car, a door lock is provided in a door of the car so as to prevent the car from being robbed or to prevent the car from being invaded and devices inside the car being damaged when the car is not used. In the related art, locking or unlocking of the door lock is performed by inserting a key for engine start-up into a key hole. However, a so-called keyless entry system that performs unlocking or locking of the door lock by operating a switch of a portable device instead of inserting a key into a key hole is used from the viewpoint of convenience. Further, in recent years, a so-called passive keyless entry system that automatically performs locking or unlocking of a door lock if a predetermined portable device is carried and located in a predetermined region instead of a switch of the portable device being operated is used. Further, the passive keyless entry system is caused to have an electrical authentication function regarding start-up of an engine, and the engine is prevented from starting when authentication is not successful between a car body and the portable device for high security.
In an operation of the keyless entry system, when a person carrying a portable device registered in an in-vehicle device mounted in a car in advance approaches the car, a request signal that is a low frequency signal containing a start-up signal is transmitted from the in-vehicle device. Also, when the portable device receives the request signal, the portable device responsively transmits an answer signal that is a high frequency signal containing an instruction signal, and the in-vehicle device controls a controlled device according to the instruction signal contained in the answer signal when the in-vehicle device receives the answer signal. This control, for example, unlocks the door of the car or starts up the engine of the car, and thus, a car driver comes to be able to drive the car.
The keyless entry system in which such a series of operations are performed includes a keyless entry system having a function of contributing to start-up of an engine when a portable device is in a car's interior, and a keyless entry system having a function of contributing to locking or unlocking of a door lock when a portable device is in a car's exterior. Further, there is a keyless entry system having a function of preventing a door from being locked when the portable device is in the car's interior so as to prevent the portable device from being misplaced in the car's interior.
In such a keyless entry system, it is important to detect whether the portable device is in the car's interior or the car's exterior with high precision. A keyless entry system in which a plurality of antennas transmitting a request signal are arranged in the car's interior or in both the car's interior and the car's exterior is known as the keyless entry system that performs the detection or the determination. Also, according to this type of keyless entry system, since the request signal is transmitted from the in-vehicle device through the plurality of antennas, transmission and reception of a radio signal can be reliably performed between the in-vehicle device and the portable device if the portable device is in the car's interior or within a predetermined range outside the car. Accordingly, it can be accurately determined whether the portable device is in the car's interior or the car's exterior.
In the related art, in the portable device in these keyless entry systems, detection sensitivity of the signal is always set to high sensitivity in order for the request signal transmitted from the in-vehicle device to be necessarily detected. Therefore, there is a problem in that consumption of a battery for a power supply built in the portable device increases. In order to solve this problem, an invention of a keyless entry system in which reduction of battery lifespan is suppressed by switching the detection sensitivity is disclosed in Japanese Unexamined Patent Application Publication No. 2013-083051. A schematic configuration of a vehicle control system 900 described in Japanese Unexamined Patent Application Publication No. 2013-083051 is illustrated in FIG. 6.
The vehicle control system 900 includes a smart key 910 operated by a built-in battery and carried by a user, and a collation ECU 950 mounted on a vehicle 980, and is configured so that wireless communication is performed between the smart key 910 and the collation ECU 950. Also, the smart key 910 periodically switches between predetermined normal sensitivity and high sensitivity at which a detection area of a transmission signal is wider than that at the normal sensitivity as detection sensitivity for detecting a transmission signal transmitted from the collation ECU 950. Accordingly, since it is not always necessary to detect the transmission signal with high sensitivity, current consumption of the battery can be reduced. As a result, reduction of lifespan of the battery built in the smart key 910 can be suppressed.
However, there are the following problems with the vehicle control system 900 described in Japanese Unexamined Patent Application Publication No. 2013-083051. A case in which it is necessary for the smart key 910 that is a portable device to detect the transmission signal transmitted from the collation ECU 950 that is an in-vehicle device includes only a case in which the smart key 910 is in the vehicle's interior or near the vehicle. Therefore, in other cases, it is not necessary to detect the transmission signal from the collation ECU 950. However, in the vehicle control system 900, periodic switching occurs between the normal sensitivity and the high sensitivity regardless of a position of the smart key 910. Therefore, a period of the high sensitivity in which current consumption of the battery increases is unnecessarily long. As a result, reduction of lifespan of the battery built in the smart key 910 cannot be sufficiently suppressed.